A window comparator monitors an input voltage and provides a digital output which indicates whether the input voltage is within or without a pre-defined voltage window. For reasons of compatibility, the input voltage is normally a differential input voltage. The window comparator is quite difficult to implement in a standard analog circuitry. The differential input voltage has to be converted to a single ended signal using an instrumentation amplifier. The single ended signal needs to be compared with the reference voltage that sets the window width and also with the inverted reference voltage. This means an inverting amplifier and two comparators are also necessary to implement the window comparator. So, the conventional approach requires at least four units, i.e. level shifter, inverting amplifier, and two comparators, each of these units having an offset and a gain error. Because of the amount of involved circuitry, trimming or calibration is required to achieve the demanded accuracy.
The total sum of these errors causes a very loose specification for offset and hysteresis. In order to prevent oscillation at the output of the window comparator when the input voltage exceeds the limits of the predetermined window, a hysteresis must be added. But if a hysteresis is required, things get even more complicated.